Tales from the Atomic Age

Paul W. Frame

Fermi Strikes Gold and Hevesy Invents Neutron Activation Analysis

This story appeared in the April 1997 issue of the Health Physics Society's Newsletter

The discovery of artificial radioactivity was forecast by H.G. Wells (1914) to take place in 1933. Frederic and Irene Joliot-Curie almost obliged Wells: two weeks into1934 they produced radioactive 30P by bombarding aluminum with alpha particles. In Rome, the theoretician Enrico Fermi recognized that artificial radionuclides could be produced more efficiently with neutrons than alphas since neutrons striking a nucleus have no coulombic barrier to overcome. So he sent his collaborator, Emilio Segrč, off with 1000 dollars and the instructions: "Get all of the elements in Mendelyev’s table" (Latil 1966; Regato 1985). Soon, Fermi had bombarded over 60 elements and characterized the induced activities, e.g., striking gold produced radioactive material with a one to two day half-life. Impressed, Rutherford sent Fermi a telegram reading, "I congratulate you on your successful escape from the sphere of theoretical physics" (Regato 1985). Others were equally impressed and Fermi was awarded the 1938 Nobel Prize. Suspicious that his Nobel Prize medal wasn’t solid gold, Fermi weighed it under water in his bathtub to measure its density. Having satisfied himself that it was indeed gold, he might have kept the medal with his money and other valuables in a hole he that had dug in his basement (Marshall Libby 1979).

During the aforementioned investigations, Fermi hadn’t left the procurement of elements entirely up to Segrč; in October of 1934, he wrote George de Hevesy in Copenhagen for help obtaining samples of the rare earths (Levi 1985). Hevesy responded with the names of people whom Fermi might contact, but he didn’t let on that he had what Fermi wanted—a collection of rare earths from the master chemist and founder of the Auer company, Carl Auer von Welsbach. Hevesy wasn’t about to do Fermi any favors, he intended to activate the rare earths himself.

Shortly after Hevesy’s assistant, Hilde Levi, had assembled the equipment for counting the activated samples, Hevesy had to leave on a trip. To ensure that Levi had something to do, he gave her Auer’s dysprosium and said something like "Well, take this one and try it" (Levi 1986). Levi dutifully exposed the sample to neutrons, but the GM, connected to a mechanical telephone register to record the counts, indicated absolutely no activity. Disappointed, she put the sample and its holder away and forgot about them. Later, in need of the holder, Levi retrieved and cleaned it. Then "like a good student" she counted the empty holder. The register "rattled away" and the count rate even seemed to be increasing! Running to Otto Frisch, she asked, "Frisch, have you ever heard of an increasing radioactivity with time?" After puzzling over the matter, they realized that the original sample had been so hot that the counter went completely dead. Then, with an almost invisible amount of dysprosium on the cleaned holder, and a decaying activity, the dead time dropped and the count rate went up (Levi 1986).

"That’s wonderful!" was Hevesy’s response on hearing Levi’s report. The episode triggered the realization that the half-lives and magnitudes of induced activities could be used to identify and quantify trace elements!

The large dead time of the mechanical counting registers that would later lead Robley Evans to invent the count rate meter (see "Tales . . . " July 1996 Newsletter) led Hevesy to invent an analytical method of unparalleled power: neutron activation analysis.

References

Latil, P. Enrico Fermi, the man and his theories. Paul S. Eriksson, Inc.; New York; 1966.

Levi, H. George de Hevesy. Adam Hilger Ltd; Bristol; 1985.

Levi, H. Semi centennial lecture. Modern trends in activation analysis, 7th International Conference; June 23, 1986.

Marshall Libby, L. The uranium people. Crane Russak; New York; 1979.

Regato, J.A. Radiological physicists. Am. Inst. Physics; New York; 1985.

Wells, H.G. The world set free. New York: E. P. Dutton & Co.; 1914.